Frequency modulation receiver



Aug. 14, 1945. G. L usELMAN QUENCY VMODULATIQN RECEIVER FEE 2 sheets-sheet 1 Filled Aug. 21, 1942 INVENTOR l I @auf l.l

muws@ ATTORNEY Aug. 14, 1945. G. l..- ussELMAN FREQUENCY MODULATION RE`GIVER Filed Aug. 21, 194:2

2 Sheets-,Sheet 2 Y INVENTO'R iufm' L Msn-4 MAN. -v

5235 l ddl ATTORN EY Patented Aug. 14, 1945 FREQUENCY MoDUEAfrIoN RECEIVER George L. Usselman, Port Jefferson, Y., assignor to Radio Corporation of America, a corporation of Delaware Application August 21, 1942, Serial No. 455,567

27 Claims.

The present application relates to timing modulation, such as, phase and frequency modulation and particularly to a new type of timing modulation receiver with automatic frequency control means for keeping the receiver in tune with the incoming wave.

In known timing modulation receivers the deviations in accordance with signals from the mean or base carrier frequency are converted to amplitude variations and the latter are detected. My receiver is different from most timing modulation receivers in that local oscillations of the desired frequency are produced and mixed with the received wave and the direction of phase rotation of the beat note obtained, when the received signal is beat with oscillations from a substantially fixed frequency oscillator, is detected.

In a preferred embodiment the mean or base frequency of the modulated wave when subjected to the demodulation process is the same as the frequency of the oscillations provided by the oscillator. The wave and oscillations are beat one against the other and phase deviations of the beat note (with respect to zero beat) are of a direction and size-dependent upon the frequency relation of the wave and oscillations and the extent of difference therebetween respectively. Since in my system detection is of the direction of phase rotation, a limiting action is derived and amplitude variations are eliminated by the inherent operation of the system.

In timing modulation the wave is varied in phase or frequency about a mean or average frequency which can in general be considered the mean between the deviation limits. In space wave signalling a carrier is modulated in frequency alternately at signal frequency between two frequency values and the time during which the wavel is deviated in one direction-may be greater than the time it is deviated in the other direction so that the average frequency may or may not the center frequency. It is essential that the receiver be tuned to the center frequency or else the response will be unbalanced.

The receiver of my invention is particularly adapted to use in spaced wave signalling and in all waver length or timing modulation systems.

It is quite important to keep the receiver tuned properly to the incoming signal. This function may be done by hand but that requires constant attention.I A much better method is to automatically keep the receiver in tune. Automatic tuning control of amplitude modulation receivers has been quite well worked out.

Ordinary methods of automatic tuning control of frequency modulation receivers have been disclosed'previously. These methods usually have the defect that the receiver'is tuned on or toward either space or mark, whichever is greater than 50 per cent of the signal time; that is, when known automatic tuning means are used on spaced wave keying they have a tendency to tune to the average frequency rather than the mean frequency.

The present invention will keep the receiver tuned in the proper frequency band irrespective of the lengths or weight of space or mark signals.

In describing my invention in detail, reference will be made to the attached drawings wherein two specic embodiments of my invention are shown .in Figures 1 and2. IThe arrangements of both gures are in general the same and operate in substantially the same manner. In Figure 1 thereceiver only is shown. In Figure 2 a second embodiment of the receiver is shown and automatic tuning means is also provided. The tuning means of Figure 2 Ais to be included with the receiver of Figure 1, if desired.

In Figure l wave energy, such as wave energy the frequency of which is shifted between two values by telegraphy signals separated in the frequency spectrum by several thousand cycles is picked up in the aerial 2. The wave may be shifted between a value representing the marking frequency and a second value representing, the

' spacing frequency. The wave is amplified and heterodyned to lower correspondingly spaced frequencies in unit 4'. The unit 4 may also include l. intermediate frequency amplifiers and theenergy of intermediate frequency is fed through coupling condenser 5 to the grids 6 and 8 of tubes VI and V2 respectively by way of phase advancing condenser C cooperating with phasing resistance 9 and phase retarding inductance L cooperating with phasing resistance l0. The condenser C and resistance 9 are such as to advance the phase Aof the wave in its two positions, while the inductance L and resistance Ill'retard the phase of the wave in its two positions. Positive potential for these two electrodes is supplied through resistors 9 and l0 a point on which is connected by blocking and radio-frequency bypass condenser l2 to ground.

Oscillations of constant frequency developed by known appropriate means in unit F are fed through transformer T and parasitic suppression resistances I4 and I6 to the control grids -l8 and 2l) of tubes VI and V2. The screen grids 22 and 24 are connected to a source of direct current which is shunted by bypass condenser 26. The

negative potential source (not shown) for control grids I8 and 20 is shunted by bypass condenser 28.

The anodes 30 and 32 are connected by radiofrequency filter circuits 34 and 36 and impedance couplings 44 and 46 to -the control grids 52 and 54 of the tubes 56 and 58. The anode 60 of tube B is connected to the source 62 through resistance 64 and relay winding 66. A similar connection runs from the anode 10 of tube 58 through resistance 14 and relay winding 16. The cathodes of tubes 56 and 58 are connected together and to a positive point on source 62, this section of which is shunted by bypass condenser 5l. The windings 68 and 16 cooperate with armature K and two contacts to complete two circuits having a commonleg. For example, these circuits may be connected to an indicator or recorder, or may control or key a tone generator connected to a recorder or indicator of othertype.

The anode 60 of tube 56 is connected to the grid 54 of tube 58 through resistance 6I. A similar connection runs through resistance TI between the anode 10 of tube 58 and grid 52 of tube 58. The tubes 56 and 58 with their crossconnected anodes and grids'form a tripping circuit wherein current is tripped from one tube to the other by the excitation voltage. In the absence of applied voltage on the grids, the cir cuits and tubes are in a state of unbalance and unequal currents which may be large and small (or zero) flowin their anode circuits.

If the grid of one tube, say tube 56, is made less negative or more positive this tube draws current and the current flowing in its anode circuit produces in the anode resistance 64 a negative potential which is transferred to the grid 54 of the other tube 58 to make it draw less current. 'I'his switches the current to the first tube 56 and from the second tube 58. This condition remains and pulls the contact K to one side until an applied potential on the grid of the other tube 58 starts it to draw current and produce a potential in resistance 14 and this acts through the cross connections to switch the current to the other tube 58 and from the first tube 56. Tube arrangements of this type are known in the art and have been disclosed more in detail in J. L. Finch U. s. Patent No'. 1,844,950.

In Figure l the inductances Ll and L2 with the Condensers CI and C2 are for the purpose of preventing radio-frequency current from reaching the tripping circuit. Resistors RI and R2 are for damping out parasitics. However, they may be omitted 'where screen grid tubes are used.

The arrangement of Figure 2 is similar in many respects to the arrangement of Figure 1 but differs therefrom` in otherrespects. The modulated wave of reduced frequency is fed in phase displaced relation to the control grids I8' and 20 and the point on resistors 9 and Il! is supplied by a negative potential. The source F is coupled to the grids 6 and 8 by a coupling condenser 1 and a radio-frequency impedance LI.

The anodes 30 and 32 are connected by resistors 3| and 33 respectively to the primaries of transformers TI and T2 and through these primaries to a source of anode potential. Anodes 30 and 32 are grounded for radio frequencies by Condensers 35 and 31- connecting the anodes 30 and 32 respectively to ground. Condensers 38 and 4l serve to balance the tubes and transformer inputs withrespect to ground. They are also part of the radio-frequency filter. The secondary windings of transformer TI and T2 are coupled by resistances 43 and 45 to the grids of the electron systems of the tripping circuit. In both modifications the tube systems of the trip ping circuit may be included in a single enn velope or in separate envelopes.

The receiver of Figure 2 is essentially the same as the receiver of Figure 1 and differs therefrom mainly in the manner of coupling the tripping circuit including tubes 56 and 58 to the output of the phase detectors VI and V2. In Figure l capacity coupling is used, while in Figure 2 transformer coupling, is used.

In order to tune the receivers automatically a tuning means as shown in Figure 2 may be used. This tuning means includes an amplifier tube V5 having its control grid 90 coupled to the output of the phase detector at, say 3|, and its anode 94 coupled to a load resistance 98 points on which are coupled by a high pass filter Z to a rectifier V6, the output of which is in series with a relay (#2 relay) armature and one or the other of the two windings of a relay (relay #3) the armature of which cooperates with two contacts in the field winding circuits of a reversiblefield motor M, the shaft of which drives a tuning r'eactance Y in the oscillation generator circuit of the receiver 4. The #2 relay windings are connected to the output of the tripping circuit including tubes 58 and 58 say in shunt to the windings 66 and 16 of the output relay identified here as relay #L My invention, it is believed, will be understood by those skilled in the art from the above description. However, a statement of the operation will now be given. In doing so, reference will first be made to Figure 2. Assume that the frequency modulated carrier wave received on antenna 2 is shifted through about and -2500 cycles, or a total of 5000 cycles. This signal car-v rier wave is amplified and heterodyned to a lower frequency, say about 1,000,000 cycles average o1' mean frequency, which may again be amplified before it leaves the unit 4. The 1,000,000 cycle wave is correspondingly shifted plus and minus 2500 cycles the two frequencies representing mark and space conditions. The frequency modulated or shifted intermediate frequency wave (1,000,000) cycles is delivered to the control grids I8 and 20 of detector tubes VI and V2 through phase shifter elements C!)v and L10 respectively. The phase shifter condenser C with resistor 9 advances the phase of the excitation, that is, either mark or space frequency, reaching the control grid of tube VI, while the phase shifter inductance L with resistor l0 retards the phase of the excitation, that is, either mark or space frequency, reaching the grid of tube V2. Consequently, there is a phase difference between the excitation potentials on the control grids ofv the two detector tubes VI and V2. This phase difference is preferably in the order of 45 to 90.

The frequency at the output of source F is substantially constant vand preferably is the same frequency as the carrier or center frequency delivered by unit 4. The output of source F is delivered in parallel to the third grids 6 and 8 of detector tubesV VI and V2 through resistors I4 and I6. I'he resistors i4 and I6 are used to prevent parasitics.

Now the keyed oscillating potentials from the .units 4 and constant frequency potentials from F, both being impressed on the grids of detector aasasco 3 tubes Vl and V2, cause a beat frequency to appear in the outputs of these detectors tubes. This double detector including tubes Vl and V2 not only again reduces the 'frequency but also detects the direction of phase rotation ofv the beat note which now represents the signal. This beat frequency is of the same frequency, say 2500 cycles, in each detector tube but `there is a phase difference determined bythe values of the phase shift elements C9 and LIB. If, during the mod-.- ulating cycle, the frequency of the output of unit 4 is higher than that of unit F, then the phase of the beat oscillations of tube Vl will lead that of tube V2. Also, if the'frequency of -the output of unit 4 is lower than that of unit Fl, then the phase of the beat oscillations of tube V2 will lead that of tube Vl.

With respect to each detector'its Abeat note isabout 2500 cycles for mark and for space but shifts from one adjusted phase position through zero phase and frequency to a new opposed phase position each time the carrier is `shifted from marking frequency to spacing frequency and vice versa. Thus, for mark we havea 2500 cycle beat note of `one phase rotation andfor space a 2500 cycle beat note of opposite phase rotation.

These beats which are of zero frequency When the output of F and the mean frequency of 4 are the same are fed to transformers Tl and T2 and thence to the tripping tubes 56v and 58.

The tripping circuit includingthe tubes 56 and 58 will be actuatedby the beatoscillations delivered from detector tubes Vl and- V2 through transformers' TI and T2 '.respectively. In the tripping circuit only the side E orD carries blocks of current at one time. For example, if the ,fre-

vquency from unit 4 is higher than that from unit Ffdetector tube Vl, wherein the beat note of leading phase appears, will trip the E side of the tripping circuit. yThis will then carry a block of direct current during a time `(beat.frequency cycle) equivalent to, say 60%. Then detector tube V2 trips tube 5B so that the D. side of the tripping circuit carries a block of direct Current equivalent to 300. Consequently, the. relayK will be pulled, say to the lower contact. Now,

which time the noteof lagging phase acts on the other side toftrip' the current to the other 'side where it stays for BOO/1360 of a'cycle of the beat note so that the average current through the said other side is the greater.- This lhas been fully explained in my Patents No. 2,018,820-and No. 2,044,749. n

Now it can be seen that as the signalfrequency is modulated up and down, as it Would be in the-case of spacing wave keying, the-phase relations of the two beat notes from therdetectcr tubes Vl and V2 will cause the .keying relay K tooperate in accordance with the keying of the transmitter.

In the embodiment shown vthe `armature K cooperates with two contacts designatedl R and L'to key the output of a tone generatorl to convey the intelligence over wire lines to a'trafi'ic office. The said keyed tone may be used for other purposes, such as, for example, to key a transmitter to thereby relay the intelligence to a thirdpoint.

Now it may be seen in this assumed case, if the receiver or transmitter'frequeno'y drifts over 2500 cycles, phase reversals in the second detector tubes Vl and V2 will not take place andthe receiver will not function. If 5000 cyclesplus and minus frequency deviation is assumed; then a frequency drift of 5000'cycles can take-place before the receiver fails to function, etc. .This frequency'driftis' compensated for bythe present invention which' automatically keeps the receiver tuned in the proper frequency band.

The automatic 'frequency control means will now be described:

v Excitation is obtained for the grid'SflV of am.- plifier tube V5 from thev output of one of the second detector tubes, Vl, in this case. The am.-

by similar sequence, if the frequency output from unit 4 vis lower than that from unit 11k-detector tube V2, wherein the beat note of leadingphase appears, will trip the side `D of the `tripping circuit so that this tube takes the current for the shortest time and then the upperside of the tripping circuit will carry the long vblocks of current, and, consequently, it will carry the more average direct current and relay K willbe pulled upward.

Stated in another way, suppose that the phase shifters C9 and LIU were such as to produce1180 or phase opposed relation between the beat notes fed from tubes Vl and V2 to the tripping circuit. Then the two sides of the tripping tubes each draws current about one half the beat note cycle and their average currents are equal, andthe armature of K will remain in the neutral posi.- tion. This is because one side is excited by the leading beat note and current is tripped to 4that one side for half a cycle at which time the beat note of lagging phase excites theA other side of the tube and current isl then tripped to the other side for half a cycle of the beat note. However, here the phase displacement is, say so that the beat note of leading phase-trips the current to one side and it flows in that side .during 60/360 vof a cycle of the beat note frequency 'at plified` detected signal frequencyenergy from stage V5Vis fed into high pass filter Z. Theoutput 'of filter Z is fed to rectifier `V6=by way lof transformer T3. We will assume thatlter Z will pass through energy whose frquency is above 2500 cycles. Consequently, if the receiver drifts inl either direction so that the mark or space detector output frequency is over 2500 cycles, the filter Z will pass this frequency and itfwill be rectified in rectifier V6. This rectified current is delivered tothe tongue KI of relay #2. The winding ofrelay #2 is connected in parallel with the winding of relay #L which is in the output` circuit of tubes V3, V4 as shown. Depending upon the phase rotation of the signal, the relay #2 'will be operated to left L or rightR. `Rectified current is at this time flowing in rectifier V6 and that half of the winding of relay #3 put in circuit by movement of armature KI.-This causes similar operation of the armature K2 of relay #3, and the motor M is operated counterclockwise or clockwise. Motor M turns a condenseror variable inductalle in the frequency controlled heterodyned oscillator which varies the Oscillator frequency. Consequently, the average intermediate frequency is changed. Y

Now, if the directionof this sequence of .operation is correctly made, the tuning controlapparatus will automatically bring the receiver back-in tune for proper operation. In other words, the high pass filter Z determines when and how much the receiver should be retuned and the relay #2 through the second detector Iald, tubes 50 and 58 determines the sense or difrection inlwhich this retuning must be made. It may be seen now that the weight vorrel- @tive lensthsfof the mark and .Space elements 0f the Signal have nothing to do with the autematic tuning of the receiver. If either the mark or the space part of the signal begins to edge out of the operating band the tuning of the receiver is automatically corrected to insure proper operation by bringing the receiver back into the operating band.

In the condition where the receiver is so much out of tune that there are no phase reversals in the second detector output` (that is, tubes VI, V2 and 56 and 58), the relays #l and l#2 will be pulled constantly to one side makingcontact continuously on one side of the relay. Then since filter Z and transformer T3 and rectifier V6 are supplying current to the tongue of relay #2, the automatic tuning Vprocess will be continuous until the receiver is brought back in tune.

In the condition where the receiver is only partly or slightly out of tune, or, in other Words, where the edges of the marks or spaces-are ybe.- yond the frequency band limits,` there will still be phase reversals and the relays #I and #2 will vibrate, moving the tongue alternately between the twov contacts. However, the filter Z and transformer T3 and rectifier V6 will only rectify and supply current when the relay tongueis touching one contact. This'will bethe contact which causes the control system to correct the receiver frequency. In other words, the rectified current from T3-V6 in this case is synchronized with'the motion of the tongue of relay #t2v so as to correct the out-of-tune condition of the receiver.

Relay #3 is of a type in which the tongue does not touch either contact when n'o current is flowing through its coils. Parts of thecircuit not explained'are assumed to be easily understood because the symbols used are standard.

This invention can be used for automaticltuning'control on the conventional type frequency modulation'receiver by employing a sourceas at F 'and tubes VI, V2, and 56, 58, in addition to the'automatie tuning apparatus shown in this invention. By using special transformers for TI-TZ, tuning correction can be obtained'over a wide range. -f

Many modifications may be made in this receiver tuning control and. still be within the scope of this invention. For instance, any number of amplifiers may be used in place of tube V5. Any

suitable type of filter may be used for unit Z.

Any suitable type of rectifier may be used in place of T3-V6, such as, oxide rectifiers. The foregoing stages may also beof the push-pull type.

4'The keying relay 66, 16 and K may be replaced with any desirable response equipment like a transformer and a speaker unit or a reversing direct current motor, etc. The output leads of relay #I may be in the circuit of recording apparatus.

The tripping circuit may also include inhibiting means, such as those described in my Patents Nos. 2,018,820 and 2,044,749, to stop operation of the tripping circuit whenv no signal is being received. 'l A Receiver circuits of this type may be used for holding facsimile and television receivers in synchronism with the carrier or average frequency of the transmitter. f

This receiver may be used to operate a vmotor to steer a boat or airplane, in which case one frequency would turn the rudder in one direction and the other frequency would turn the rudder in theV opposite direction. y

The phase rotation detector and the tripping circuits in this receiver act as both signal amplitude limiter and frequencymodulation detector; It is at the same time an amplitude and a frequency modulation limiter because it voperates on the change of phase rotation of the detected signal andits operation is substantially independent of'changes'in amplitude and frequency deviation vof the signal.

The oscillators inthe heterodyne unit l and the comparison unit Fin Figure 1 maybe very constant' frequency crystal oscillators and it would also be desirable for the Aaverage or carrier frequency of the transmitter to be very consta'nt. Otherwise, the oscillators'in units 4 and F could be provided with Vernier hand adjustments,'or with -automatic tuning means as shown in Figure 2. l

The receiver of Figure 1 operates substantially in the same manner in which thefreceiver 'of Figure 2 operates, excepting the automatic tuning. l

I claim:` I

1. The method of simultaneously limiting the amplitude and demodulating wave energy the frequency of which is shifted Ifrom one frequency value for one signalling condition to another frequencyA value for another signalling condition which includes the steps of producing oscillations of substantially fixed frequency intermediate the frequencies between which said wave energy isshiftedl beating said oscillations and wave energy one against the other to produce a beat note of 'substantially flxedfrequency the phase or frequency rotation of which is in one direction or the other, ldepending on the relative positionsrof said wave energy and said oscillations in the frequency spectrum, and detecting the phase changes only of said beat note.

2. The method of demodulating wave energy' the frequency of which is shiftedifrom one frequency value for one signalling condition to another ffrequency value for another signalling condition which includes the steps of producing oscillations of substantially fixed frequency intermediate the frequencies between which said Wave energy is shifted during the said signalling conditions, beating said oscillations and wave energy to produce a beat note the phase or frequencyl rotation of which is in 'one direction or the other, depending on the relative positions of said wave energy and said oscillations in the frequency spectrum, and producing indications of the direction of rotation of said beat note.

3. The method of demodulating wave energy the frequency of which is shifted from one 'frequency to another frequency in a, manner to represent signals, which includes the steps of producing oscillations of substantially fixed frequency substantially equal to themean value of the frequencies between which said wave energy isv shifted, producing a phase displacement of -less than between two pontions of said shifted wave energy, beating both of the wave energy portions with said oscillations to alternately; derive two beat; notes of -like frequency, and of phases which depend on the relative positions of said wave energy and said produced oscillations,.and producing currents characteristic of the phase relation of said beat notes.

4.' The method of demodulating wave energy,

the frequency of which is shifted from one Ifrequency to another frequency in a manner to represent signals, which includes the steps of producing oscillations of substantially fixed frequency vequal to the mean of the frequencies between which said wave energy is shifted, advancing the phase of a portion of said shift'ed wave energy retarding the phase of another portion of said shifted wave energy, beating the wave energies of advanced phase and of retarded phase with said oscillations to .alternately derive two beat notes, and producing currents characteristic of the phase relation of said beat notes.

5. In a system for demodulating a signal carrying wave which alternates between a spacing frequency and a marking frequency separated by a band of frequencies, a converter tube and circuit, a source of oscillations of a frequency intermediate the marking and spacing frequencies, a coupling between said source of oscillations and said `converter tube, a circuit for impressing said signal carrying wave on said converter tube whereby a beat note is produced by said converter tube thephase of which alternates in direction of rotation depending on the frequency relation of the oscillations and the wave, and a phase detector coupled to said converter circuit. y

6. The method of automatically tuning a receiver, responsive to signals comprising a carrier shifted in frequency from a first frequency to a second frequency, to a mean frequency intermediate said first yand second frequencies irrespective of unequality in the division of time during which said carrier is at said first and second frequencies which includes the following steps,

generating oscillations of substantially fixedl frequency substantially equal to said mean frequency, beating said shifted carrier with said oscillations to produce a beat note the frequency of which, depends on the frequency relation of said first and second frequencies to said mean frequency and the direction of phase rotation of which reverses as said carrier is shifted, producing current characteristic of the direction of phase rotation of said beat note and controlling the mean frequency ofsaid carrier in accordance with said produced currents when the beat note is of a frequency greater than a selected value.

7. The method'of automatically tuning a heterodyne receiver, responsive to signals comprising -a carrier shifted in frequency from a first frequency to a second frequency, to a mean frequency intermediate said first and second frequencies irrespective of unequality in the division of time during which said carrier is at said first and second frequencies which includes the following steps, generating oscillations of substantially xed frequency substantially equal to said mean frequency, beating said shifted carrier with said oscillations to produce a beat note the frequency of which depends on the frequency relation of said rst and second frequencies to said mean frequency and the direction of phase rotation of which reverses as said carrier is shifted, producing current characteristic of the direction of phase rotation of said beat note, controlling the mean frequency of said carrier in accordance with said currents, and inhibiting said controlling action when the beat'note is of a frequency less than a selected value.

8. 'Ihe method of receiving signals comprising a carrier shifted in frequency from a first fre-- quency to a second frequency substantially equally spaced with respect to a mean frequency intermediate said first and second frequencies and preventing unequality in the division of time during which said carrier is at said first and second frequencies from preventing such recep- 'tion which includes the -following steps, generating oscillations of substantially fixed frequency substantially equal to said mean frequency, beating saidv shifted carrier rwith said oscillations to produce a beat note the frequency of which depends on the frequency relation of saidifirst and second frequencies to said mean frequency and the phase rotation of which reverses as said carrier is shifted, detecting the phase of said beat note to derive the signals, producing direct current the direction offiow of which is characteristic of the direction of phase rotation of said beat note, and controlling the mean frequency of said carrier in accordance with said produced current when the beat note is of a frequency greater than a selected value. i

9. In a receiver of carrier current keyed from a first frequency to a second frequency in accordance with signals, tunable means for heterodyning said keyed carrier current to a'lowerfrequency to produce current of lowerfrequencies correspondingly keyed from a, first toa second frequency, a source of oscillations of. afrequency intermediate said lower first and second frequencies, means for beating oscillations from said source with said lower frequency currents from said heterodyne to produce a, beat note 'the direction of phase rctationof which depends .on the position of said current of lower frequency as keyed relative to oscillations from said source of oscillations, means for producing acurrent characteristic of the direction .ofphase rotation of the beat note for reproducingsaid signals, other means for producing a direct current when the frequency of said beat note exceeds ,a predetermined value, an automatic tuning control device coupled with said tunable means, an oper-.- ating circuit including circuit closing contacts for said device, a connection betweensaid other means and at least one of said contacts, and a relay having Ian armature associated with at least one of said contacts for completing said operating circuit and supplying thereto. the direct current produced by said other means when said armature is actuated, said relay having two armature actuating windings coupledto said first named current producing means.

10. In a receiver of carrier current keyed from a first frequency to a second frequency in accordance with signals, a tunable oscillator for heterodyning said keyed carrier to a lower frequency to'` produce current of lower frequency vGorrespondingly keyed from a. first to a second frequency, a source of oscillations of,` a frequency intermediate said lower first and second frequencies, phase detecting dischargesystems for beating oscillations from said source with said lower frequency currents from Said heterodyne to produce a beat note the direction of phase rotation of which depends on the keyed position of 'said current of lower frequencyv relative to oscillations from said source, a phase rotation detector for-producing a current the directionof flow of which is characteristic of the direction of phase rotation of the beat note, a high pass filter and a rectifier for producing adirect current when the frequency of said beat `note exceeds' a predtermined value, `a variable reactance in' said tunable oscillator, a control means for said variable reactance, said control means having an actuating circuit including contacts when'closed connect said actuating circuit to said'rectifier, and a contact closing relay having' a` winding coupled to said phase rotationdetecton and having an armature associated with said contacts.

11. In `a system for receiving .a `carrier keyed inaccordance with signalsfrom' a first'frequency to a second frequency, a heterodyning system including a tunable source of oscillations and having Aaninput and an output, the frequency of the energy of the output being determined by the tune of the oscillator, means for impressing said wave energy on the input of said heterodyne system, means for selecting side band currents correspondingly keyed between first and second frequencies from the heterodyne system, a source of oscillations of substantially constant frequency of a value intermediate said first and second frequencies of said side band currents, a mixer tube coupled to said source and to the output of said heterodyne system,. said mixer tube having an output wherein a beat note is produced of a frequency dependent on the relative frequencies of said side band currents and oscillations from said source and of a phase rotation which reverses as said side band current is keyed, a. tripping circuit coupled to the output of said mixer, a utilizing circuit coupled to said tripping circuit, a motor connected with a, tuning element in said heterodyne oscillator said motor having windings the direction of flow of current through which determines the direction of rotation of the motor, a high pass filter coupled to the output of said mixer tube, a relay having windings coupled to the output of said tripping circuit, said relay having an armature for controlling the position of an additional Contact cooperating with other contacts, a rectifier coupling the output of said high passv filter to the said additional contact controlled lby the armature of said last named relay 'and a source of potential adapted to be connected to a winding of said motor to supply direct current thereto when the amature of said relay moves said additional contact against one of said other contacts.

12. In a receiver of carrier energy, the frequency of which is keyed from a first frequency to a second frequency in accordance with signais, a tunable oscillator for heterodynng Said carrier to a lower frequency, a pair of electron discharge devices having input and output electrodes, a, source of oscillations of a substantially constant frequency intermediate said first and second frequencies coupled to input, electrodes of said pair of devices, a phase splitting circuit coupling said heterodyne to input electrodes of Said pair of devices, a tripping circuit comprising a second pair of electron discharge tubes having input and output electrodes with cross-coupling between the input and output electrodes of the tubespmeans coupling the input electrodes of said last pair of tubes to the output electrodes of said first pair of devices, a, relay comprising an armature and a. pair of windings coupled to the outputelectrodes of said second pair of tubes, a rst contact the position of which is controlled by said amature, an'amplifier having an input and an output with the input coupled to the output of one of said rst named pair of devices, a rectifier coupled to said first contact, a high pass filter having an input coupled to the output of said amplifier and having an output coupled to said rectifier, a second relay having an armature and having a pair of windings associated therewith and connected to other contacts associated with the said first contact the position of which is controlled by the armature of said first relay, an additional contact the position of which is controlled by the armature of said second relay, a reversi-ble motor connected to a tuning reactance in said heterodyne oscillator and field windings forV said motor connected with still other contacts associated with the ad ditional contact the position of which is 00ntrolled by the armature of said second named re` lay.

13. In means for effectively limiting the amplitude of and demodulating wave energy the fre# quency of which is shifted from one frequency for one signalling condition to another frequency for another signalling condition, a source of 0S- cillations of a, frequency intermediate said one and another frequencies,-a. converter circuit excited by said oscillations and said wave energy to produce a beat note the phase of which reverses when the wave energy is shifted from said one to said other frequency and vice versa and a, phase rotation detector coupled to said converter circuit.

14. In a system for demodulating wave energy the frequency of which is keyed between, mark and space frequencies in' combination, circuits for beating said wave energy with oscillations of la frequency intermediate said mark and space frequencies to produce a beat note the direction of rotation of which reverses as scid wave energy is keyed from mark to space frequencies and vice versa and a. phase rotation detector coupled t0 said circuits.

15, In a signal modulated carrier weve, re-` ceiver, a circuit excited by the modulated Carrier wave for producing current. the phase of which rotates and the direction of rotation of which represents the carrier wave modulations and a combined phase rotation detector and Signal link iter coupled to said circuit. f.

16. Ina signal modulated carrier wave demodulating system, a. circuit excited by the carrier wave for producing current the phase of which rotates and the direction of rotation of which represents modulation of the wave relative to its unmodulated state, a phase rotation detector coupled to said circuit, and a modulated Wave frequency stabilizing circuit controlled by the output of said detector.

17. A system as recited in claim 14 wherein said phase rotation detector also effects limita-y tion of the amplitude of the wave.

18. In a system for demodulating wave energy the frequency of which shifts between a first value and a second value in accordance with signals, a circuit for producing current the direction of phase rotation of which represents the wave shifts from said first to second'value and vice versa, a detector of the direction of phase rotation of said produced current coupled to said circuit, a circuit coupled to said detector, and controlled by its output for producing direct current the direction of flow of which reversed with changes in the direction of phase rotation of said rst mentioned produced cur-rent and u ,Sidnal relay coupled to said last named crllit., 1

19. A system as recited in claim 1.8 wherein said phase rotation detector also effects emplitude limiting of the wave and wherein a wave frequency stabilizing circuit is coupled to said last named circuit.

20. In a system for receiving. a: carrier keyed in accordance with signals from a first frequency to a, second frequency, a heterodyning system including a. tunable source of oscillations and having an input and an output, the frequency of the energy of the output being determined by the ltune of the oscillator, means for impressing said wave energy on the input of said heterodyne sys-y tem, a source of oscillations of substlltally constant frequency of a value intermediate said first and second frequencies, a `mixer'tube coupled to` said source and to Ithe output of said heterodyne system, said mixer tube `having an output wherein a beat note-is produced of a frequency dependent on the' relative frequencies of said carrier and oscillations from'said source and of a phase rotation which reverses as said carrier is keyed, a tripping circuit coupled to the output of saidy mixer, a utilizing circuitcoupled to said tripping crcuit, a'motorwconnected with a tuning element in said heterodyne oscillator said motor having windings the direction of flow of current through which determines the direction of rotation of the motor, a high pass filter coupled to the'output of said mixer tube, a relay having windings coupled to theA output of said tripping circuit, said relay having lan armature for controlling the position of an additional contact cooperating with other contacts, a rectifier coupling the output of said high pass filter to the said additional contact controlled by the armature of said last named relay and a source of potential adapted to be connected to a winding of said motor to supply direct current thereto when the armature of said relay moves said additional contact against one of said other contacts.

2l. In a receiver of carrier currents keyed from a first frequency through a mean frequency to a second frequency in accordance with signals, a source of oscillations of a frequency substantially equal to said mean frequency, a pair of electron discharge tubes having input and output electrodes, a coupling between said source of oscillations and the input electrodes of said tubes, a coupling to the input electrodes of said tubes for impressing said carrier currents thereon, phase shifting means in one of said couplings for producing a phase displacement between the excitation currents applied by the said one coupling to the input electrodes of said tubes, the arrangement being such as to produce beat notes which are of substantially constant frequency the phase rotation of which reverse as the said carrier currents are shifted through said mean frequency between said first and second frequencies, and recording means coupled with the output electrodes of said tubes.

22. In a receiver of carrier currents keyed from a first frequency through a mean frequency to a second frequency in accordance with signals, a source of oscillations of a frequency substantially equal to said mean frequency, a pair of electron discharge devices having input and output electrodes, a coupling between said source of oscillations and the input electrodes of said devices, a coupling to the input electrodes of said devices for impressing said carrier currents thereon, phase shifting means in one of said couplings for producing a phase displacement between the excitation applied by the said one coupling to the input electrodes of said devices, the arrangement being such as to produce beat notes which are of substantially constant frequency the phase rotation of which reverse as the said carrier currents are shifted through said mean frequency between said first and second frequencies, an electron discharge tube system, responsive to phase changes of said beat notes, having output electrodes and having input electrodes coupled tothe output electrodes of said devices, and recording means coupled to the output electrodes of said tube system.

23. In a receiver of carrier currents keyed from a first frequency through a mean frequency to a secondffrequencyin' accordance withv signals, a source of oscillations of a frequency substanf tiallyequal to-said mean frequency, apairr'of electron discharge tubes having input and output electrodes, a coupling between said source of oscillations and the input electrodes of said tubes, a coupling to the input electrodes of said tubes for impressing said carrier currentsv thereon, phase shifting means in one of said couplings for producing a phaseidisplacement between the excitationv vcurrents applied by the saidone coupling to the input lelectrodes ofsaidftubes, the arrangement being such as to produce beattnotes which are of substantially constant frequency the phase rotation of which reverseas the said carrier currents arev shifted through said mean frequency between said first and second frequencies, a tube system, responsive to beatnotefphase changes, having output electrodes and having input electrodes coupled to said pair of tubes, apparatus for stabilizing the mean frequency of said keyed currents, and a control circuit conditioned by energy derived from the output electrodes o-f said tube system for putting said stabilizing apparatus in operation when the beat note frequency exceeds a selected Value.

24. In a receiver of carrier current keyed from a first frequency through a mean frequency to a second frequency in accordance with signals, a source of local oscillations of a frequency intermediate said first and second frequencies, means for beating oscillations from said source with said keyed currents to produce a beat note the direction of phase rotation of which depends on the frequency of said current relative to the frequency of said oscillations, means for producing current characteristic4 of the direction of rotation of the beat note for reproducing said signals, means for producing a, control current, when the mean frequency of said carrier current shifts so that the frequency of said beat vnote exceeds a predetermined value, of a character determined by the direction of rotation of the beat note, and means actuated by said last named control current for preventing said shifts in the mean frequency of said keyed current.

25. In a receiver of carrier current keyed from a first frequency through a mean frequency to a second frequency in accordance with signals, a

vsource of local oscillations of a, frequency intermediate saidl first and second frequencies, means for beating oscillations from said source with said keyed currents to produce a beat note the direction of phase rotation of which depends on the frequency of said current relative to the fre- 'quency of the oscillations, means for producing a current characteristic of the direction of rotation of the beat note for reproducing said signals, means for producing a direct current, when the mean frequency of said carrier current shifts so that the frequency of said beat note exceeds a predetermined value, the Apolarity of the direct current being determined by the direction of rotation of the beat note, and means actuated by said last named direct current for preventing said shifts in the mean frequency of said keyed current.

26. In a receiver of carrier current keyed from a first frequency through a mean frequency to a second frequency in accordance with signals, a source of local oscillation of a frequency intermediate said rst and second frequencies, means for beating oscillations from said source with said keyed currents to produce a beat note the direction of phase rotation of which depends on the keyed position of said current, tube means for detecting lthe direction of 4rotation of said beat note, signal recording means coupled to said detector, a rectier for producingra, direct current when the mean frequency of said carrier current shifts so that the frequencylof said `beat note-exceeds a predetermined value, apparatus for stabilizing the mean frequency of said keyed current, said'apparatus including an actuating circuit with contacts which when closed includes the output of said rectifier in said circuit, and a relay having windings coupled to said detector and .having an armature associated with said contacts.

I 27. In a, receiver of carrier currents keyed from a first frequency through a mean frequency to a second frequency in accordance with signals, a source of oscillations of a frequency substantally equal to said mean frequency, a phase rotation detector excited by keyed currents and oacillations from said source for producing a beat note the frequency of which is substantially constant when said` mean frequency is constant and the direction of rotation of which depends upon the frequency of said keyed current with respect to the frequency of said oscillations, means for detecting the direction of rotation of said beat note and recording the same, means for producf ing a direct current when the said mean frequency shifts and the frequency of said beat note exceeds a predetermined value, the polarity of said direct current corresponding to the direction of rot-ation of said beat note, and means controlled by said direct current for returning said mean frequency to a predetermined value.

GEORGE L. USSELMAN. 

